1SIGNAL(2) Linux Programmer's Manual SIGNAL(2)
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6 signal - ANSI C signal handling
7
9 #include <signal.h>
10 typedef void (*sighandler_t)(int);
12 sighandler_t signal(int signum, sighandler_t handler);
14
16 The behavior of signal() varies across UNIX versions, and has also var‐
17 ied historically across different versions of Linux. Avoid its use:
18 use sigaction(2) instead. See Portability below.
19 signal() sets the disposition of the signal signum to handler, which is
21 either SIG_IGN, SIG_DFL, or the address of a programmer-defined func‐
22 tion (a "signal handler").
23 If the signal signum is delivered to the process, then one of the fol‐
25 lowing happens:
26 * If the disposition is set to SIG_IGN, then the signal is ignored.
28 * If the disposition is set to SIG_DFL, then the default action asso‐
30 ciated with the signal (see signal(7)) occurs.
31 * If the disposition is set to a function, then first either the dis‐
33 position is reset to SIG_DFL, or the signal is blocked (see Porta‐
34 bility below), and then handler is called with argument signum. If
35 invocation of the handler caused the signal to be blocked, then the
36 signal is unblocked upon return from the handler.
37 The signals SIGKILL and SIGSTOP cannot be caught or ignored.
39
41 signal() returns the previous value of the signal handler, or SIG_ERR
42 on error. In the event of an error, errno is set to indicate the
43 cause.
44
46 EINVAL signum is invalid.
47
49 C89, C99, POSIX.1-2001.
50
52 The effects of signal() in a multithreaded process are unspecified.
53 According to POSIX, the behavior of a process is undefined after it
55 ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by
56 kill(2) or raise(3). Integer division by zero has undefined result.
57 On some architectures it will generate a SIGFPE signal. (Also dividing
58 the most negative integer by -1 may generate SIGFPE.) Ignoring this
59 signal might lead to an endless loop.
60 See sigaction(2) for details on what happens when SIGCHLD is set to
62 SIG_IGN.
63 See signal(7) for a list of the async-signal-safe functions that can be
65 safely called from inside a signal handler.
66 The use of sighandler_t is a GNU extension, exposed if _GNU_SOURCE is
68 defined; glibc also defines (the BSD-derived) sig_t if _BSD_SOURCE is
69 defined. Without use of such a type, the declaration of signal() is
70 the somewhat harder to read:
71 void ( *signal(int signum, void (*handler)(int)) ) (int);
73 Portability
75 The only portable use of signal() is to set a signal's disposition to
76 SIG_DFL or SIG_IGN. The semantics when using signal() to establish a
77 signal handler vary across systems (and POSIX.1 explicitly permits this
78 variation); do not use it for this purpose.
79 POSIX.1 solved the portability mess by specifying sigaction(2), which
81 provides explicit control of the semantics when a signal handler is
82 invoked; use that interface instead of signal().
83 In the original UNIX systems, when a handler that was established using
85 signal() was invoked by the delivery of a signal, the disposition of
86 the signal would be reset to SIG_DFL, and the system did not block
87 delivery of further instances of the signal. This is equivalent to
88 calling sigaction(2) with the following flags:
89 sa.sa_flags = SA_RESETHAND | SA_NODEFER;
91 System V also provides these semantics for signal(). This was bad
93 because the signal might be delivered again before the handler had a
94 chance to reestablish itself. Furthermore, rapid deliveries of the
95 same signal could result in recursive invocations of the handler.
96 BSD improved on this situation, but unfortunately also changed the
98 semantics of the existing signal() interface while doing so. On BSD,
99 when a signal handler is invoked, the signal disposition is not reset,
100 and further instances of the signal are blocked from being delivered
101 while the handler is executing. Furthermore, certain blocking system
102 calls are automatically restarted if interrupted by a signal handler
103 (see signal(7)). The BSD semantics are equivalent to calling sigac‐
104 tion(2) with the following flags:
105 sa.sa_flags = SA_RESTART;
107 The situation on Linux is as follows:
109 * The kernel's signal() system call provides System V semantics.
111 * By default, in glibc 2 and later, the signal() wrapper function does
113 not invoke the kernel system call. Instead, it calls sigaction(2)
114 using flags that supply BSD semantics. This default behavior is pro‐
115 vided as long as the _BSD_SOURCE feature test macro is defined. By
116 default, _BSD_SOURCE is defined; it is also implicitly defined if one
117 defines _GNU_SOURCE, and can of course be explicitly defined.
118 On glibc 2 and later, if the _BSD_SOURCE feature test macro is not
120 defined, then signal() provides System V semantics. (The default
121 implicit definition of _BSD_SOURCE is not provided if one invokes
122 gcc(1) in one of its standard modes (-std=xxx or -ansi) or defines
123 various other feature test macros such as _POSIX_SOURCE,
124 _XOPEN_SOURCE, or _SVID_SOURCE; see feature_test_macros(7).)
125 * The signal() function in Linux libc4 and libc5 provide System V
127 semantics. If one on a libc5 system includes <bsd/signal.h> instead
128 of <signal.h>, then signal() provides BSD semantics.
129
131 kill(1), alarm(2), kill(2), killpg(2), pause(2), sigaction(2), sig‐
132 nalfd(2), sigpending(2), sigprocmask(2), sigsuspend(2), bsd_signal(3),
133 raise(3), siginterrupt(3), sigqueue(3), sigsetops(3), sigvec(3),
134 sysv_signal(3), signal(7)
135
137 This page is part of release 3.53 of the Linux man-pages project. A
138 description of the project, and information about reporting bugs, can
139 be found at http://www.kernel.org/doc/man-pages/.
140Linux 2013-04-19 SIGNAL(2)